I don't think there will be an abundance of tanks compared to other MMO's in TOR. However there will be a higher % of the population that has a tanking AC.

3 out of the 8 AC's have the ability to tank which is a higher ratio than most other MMO's.

I would say that Sith Jugg is the most popular choice for a couple reasons. (It is my class of choice)

A. Its the Vader/Malgus archetype, 2 very popular, badass chars.
B. Its the stereo typical "Warrior Main Tank" class, similar to other MMOsC. Force leap (Awesome)
D. Probably the biggest reason, the armor of this AC has you looking like this: